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• Plasticity can be achieved through multiple twinning modes and disclinations. • { 11 2 ¯ 6 } type extension twinning mode has been observed in mg alloys. • { 11 2 ¯ 6 } twins can be generated through reactions between twins and disclinations. • Three formation mechanisms of { 11 2 ¯ 6 } twins have been identified. Twinning-induced plasticity plays a critical role in determining the deformation behaviors of hexagonal close-packed metals (e.g., Mg and Ti), due to the lack of five independent slip systems required for a general deformation. In particular, twinning modes that can accommodate c-axis strain (tensile or compressive) are especially important, which are necessary complementary deformation modes to < a >-type dislocations. However, only limited types of extension twins, e.g., { 10 1 ¯ 2 } type twin, have been unambiguously identified in the literature, which is theoretically inadequate to accommodate a complex deformation with c-axis extension. Using topological defect theory, here we show that another extension twinning mode, { 11 2 ¯ 6 } twin, can be formed through reactions between disclinations and { 10 1 ¯ 2 } twins in Mg-alloys. Based on symmetry of the deformation space, we demonstrate that the { 11 2 ¯ 6 } twin originates from the inter-connection of correlated deformation paths, which can provide a tensile strain of 5.2% along c-axis and a compressive strain of 4.5% along 〈 11 2 ¯ 0 〉. { 11 2 ¯ 6 } twins can be formed through three formation mechanisms, i.e., direct formation, twin-twin intersection, and double twinning, which are validated by a combination of experimental characterizations and phase field simulations. Our work not only suggests a symmetry-based method to analyze twinning-induced plasticity, but also provides a new insight into the deformation mechanisms and mechanical properties of Mg-alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

• The superplastic elongation of ∼450% is achieved in low-alloyed Mg system by utilizing co-segregation of Zn, Ca, Zr and Ag atoms along the grain boundaries. • Superplastic deformation of low-alloyed Mg system can be enhanced by stress-driven grain boundary migration mechanism. • Based on disclination-dislocation reactions, a new grain boundary migration model has been proposed. • The grain boundary migration model can be used to understand the cooperative deformation mechanisms of dislocation slip and grain boundary migration. Superplastic forming, as an advanced manufacturing method, is especially important for producing complex-shaped components for Mg-alloys. Superplasticity can be achieved through grain boundary sliding at elevated temperatures in high-alloyed Mg systems, with a typical grain size < 10 μm stabilized by high densities of intermetallic precipitates. However, it is difficult to stabilize small grains and facilitate grain boundary sliding in low-alloyed systems due to insufficient precipitates. Here, by utilizing a distinctive design strategy, i.e. introducing solute segregation to enhance stability of the fine-grained structure, we obtained an Mg-1Zn-0.2Ca-0.2Zr-0.1Ag (wt.%) alloy achieving ∼450% superplastic strain. Through quasi-in-situ Electron Backscatter Diffraction analyses, we investigated systematically microstructure evolutions during superplastic deformation of the alloy at different strains. It has been found that superplasticity is achieved through the absorption of intragranular dislocations by grain boundaries, which are associated with stress-driven grain boundary migration mediated by disclination-dislocation reactions. By using a theoretical description based on stress-driven grain boundary migration and disclination-dislocation reactions, our model can capture multiple superplasticity mechanisms (e.g., grain boundary sliding and grain rotation), which establishes a fundamental relationship between the evolution of microstructural defects and the strain accommodation during superplastic deformation. Our results not only suggest an effective way to achieve superplasticity in low-alloyed Mg systems, but also provide a new insight to elucidate the nature of superplasticity mechanism by revealing the intrinsic correlation between stress-induced grain boundary migration and disclination/dislocation motion. [Display omitted] [ABSTRACT FROM AUTHOR]